Some internal combustion engines for vehicles are equipped with an electronic fuel injection control apparatus as a countermeasure for such problems as harmful exhaust components, fuel consumption ratio, etc. Among such fuel injection control apparatuses, there is one system wherein the air quantity intaken by the internal combustion engine per one cycle is generally proportional to the absolute pressure within the intake manifold. The fuel injection control apparatus of this system establishes the quantity of fuel to be injected in view of various conditions such as pressure detected by a pressure sensor, engine speed and the like.
Examples of conventional fuel injection control apparatuses are disclosed in Japanese Patent Early Laid-open Publication No. Sho 61-123729 and Japanese Patent Early Laid-open Publication No. Sho 63-189651. The apparatus disclosed in the former publication is designed such that when it is operated with high loads at warming-up time, the correction factor of an output increasing quantity is established in accordance with the warming-up state in order to prevent the air fuel ratio from becoming too thick or dense. Similarly, the apparatus disclosed in the latter publication includes a bypass air passage bypassing an inlet throttle valve, and an auxiliary air valve for regulating the air rate flowing through this bypass air passage, the idle rotating speed being controlled by means of an idle rotating speed control system of a throttle bypass system.
Also, in the conventional fuel injection control apparatus, the inlet passage pressure as one control factor for establishing the quantity of injected fuel is detected by a signal output from a pressure sensor 106 (FIG. 9) which is disposed at a connecting pipe 104 which communicates with the interior of the intake manifold 102.
However, when the pressure in the intake manifold 102 is measured as mentioned, moisture due to fuel and EGR (exhaust gas recirculation) flows into the pressure sensor 106. As a result of this moisture, the functioning of the pressure sensor 106 deteriorates.
Also, as is shown in FIG. 2, there is conventionally provided a bypass air passage 12 in order to direct a fast idle air so that it bypasses the inlet throttle valve 8 and is fed to the downstream side of the inlet throttle valve 8. The air under pressure is guided through the bypass air passage 12 (on the downstream side of an air valve 14 for opening and closing an opening 16 of the bypass air passage 12) and communicates with a pressure sensor 18 via a passage 20. Thus, it is assumed that the pressure in bypass passage 12 normally approximates the pressure in the inlet passage 6. Because the possibility of moisture due to fuel and EGR flowing into the pressure sensor 18 is small, this conventional structure is often used.
However, in the construction where the air pressure in the bypass air passage 12 on the downstream side of the air valve 14 is measured, as shown in FIG. 2, a large quantity of air flows through passage 12 when the engine temperature is low, because the air valve 14 widely opens the opening 16 of the bypass air passage 12. Thus, when the inlet throttle valve 8 is generally entirely closed, that is, at the idling operation time, the relation between the pressure P.sub.1 (absolute pressure) of the bypass air passage 12 on the downstream side of the air valve 14 and the pressure P.sub.2 (absolute pressure) of the inlet air passage 6 on the downstream side of the inlet throttle valve 8 is such that P.sub.1 is significantly greater than P.sub.2 (i.e. P.sub.1 &gt;&gt;P.sub.2 ). Accordingly, a control means (not shown) determines that the bypass pressure P.sub.1 detected by the pressure sensor 18 is large at a time when the pressure P.sub.2 is significantly less than P.sub.1. Because of the foregoing reason, the control means erroneously actuates a fuel injection valve 10 in order to enrich (needlessly) the air fuel ratio.
On the other hand, when the opening degree (angle) of the inlet throttle valve 8 becomes large, the pressure relationship becomes P.sub.1 .apprxeq.P.sub.2. Therefore, the control means performs a normal air fuel ratio controlling function.
However, when the opening degree of the throttle valve 8 is large, the air fuel ratio becomes rich in the idling state. When matching (establishing) is effected in order to bring this state into a proper state, the air fuel ratio sometimes becomes lean during operation. According to test results, the difference between the pressure P.sub.1 in the bypass air passage 12 on the downstream side of the air valve 14 and the pressure P.sub.2 in the inlet air passage 6 on the downstream side of the inlet throttle valve 8 reaches a maximum of about 28% in a multicylinder internal combustion engine, as shown in FIGS. 10 and 11. Accordingly, there is an inconvenience in that the injection quantity of fuel is needlessly adjusted and uselessly fluctuated, which degrades performance. In FIG. 2, reference character Pa denotes an atmospheric pressure.
Therefore, the object of the present invention is, for the purpose of obviating the above inconvenience, to provide a fuel injection control apparatus for an internal combustion engine in which the pressure P.sub.1 in the bypass passage is measured by a sensor, a correction factor for the fuel injection quantity is calculated in accordance with at least the engine temperature, the fuel injection is controlled by such obtained correction factor, and the quantity of injected fuel is properly controlled to improve the operation performance while protecting the function of the pressure sensor.